The present invention relates to a color masking parameter determining apparatus and more particularly to an apparatus, in a color sensor (color scanner), for determining color masking parameters to be used in transforming red (R), green (G), and blue (B) luminance signals obtained by the sensor to coordinates in the uniform color space.
In the fields of color printing, color television, and color copying machines, numbers of techniques on color signal transformation have so far been proposed. As an example, there is a technique by which an input color space (for example, the BGR system) is transformed to an output color space (for example, the CIE-L*, a*, b* system). When coordinates (L*, a*, b*) in the uniform color space are to be obtained from primary-color luminance signals (R, G, B) of an original obtained from a color-separation color scanner, the primary-color luminance signals (R, G, B) are first transformed to primary-color density signals (Dr, Dg, Db) using equation (1) and then the results obtained are transformed to the coordinates (L*, a*, b*) in the uniform color space using equation (2). ##EQU1## In equation (2), (a.sub.ij) are called color masking parameters.
In order to perform the above transform as accurately as possible, it is practiced to obtain the color masking parameters (a.sub.ij) by carrying out the method of least squares for the coordinates in the uniform color space. In concrete terms, N color patches (N: natural number) having known L*.sub.k, a*.sub.k, b*.sub.k (k=1, . . . , N) are measured by a color sensor and color masking parameters a.sub.11, a.sub.12, a.sub.13 minimizing equation (3) the value obtained by in lightness, for example, are sought: ##EQU2## This can be done by solving the simultaneous equations (4): ##EQU3##
In the apparatus for determining such color masking parameters, it is difficult to obtain the color patches to be used in obtaining the color masking parameters by sufficient number or at suitable intervals in the uniform color space. Therefore, methods not using actual color patches, but using values of simulative color patches obtained by simulation, have so far been known. For example, there is a method, disclosed in Japanese Patent Laid-open No. Sho 61-50153, by which virtual color patches are selected at regular intervals and primary-color principal densities of cyan (C), magenta (M), and yellow (Y) are transformed to primary-color luminance values (R, G, B) with Neugebauer equation used, and then color masking parameters are determined according to the values obtained.
In the above described techniques so far in use, a method to presume an ideal model is adopted in the simulation, but since the model does not conform well to actual phenomena, there has been a problem that the accuracy of the determined color masking parameters is not good. Accordingly, the present invention has as its object the provision of a color masking parameter determining apparatus, which needs production of only a small number of color patches and is capable of determining easily and economically the most suitable color masking parameters in conformity with actual phenomena.